Several methods have been proposed for the synthesis of various types of zeolites from alternative materials including fly ash, rice husk ash, metakaolin, oil shale fly ash, and other Si—Al containing feedstocks.
Many of the proposed methods for the production of zeolites from fly ash, in particular high silica content zeolites, include the fortification of the reaction mixture with an external source of silica to alter the Si:Al molar ratio to a level that will produce the desired final product.
The use of fly ash in the synthesis of high silica content zeolites such as ZSM-5 has not been as successful as anticipated due to the addition of a silica source to increase the Si:Al molar ratio. In addition, non-reactive phases such as mullite and quartz appeared in the final product.
Kalyankar et al., (2011) synthesised zeolite ZSM-5 from Indian fly ash with a SiO2/A2O3 ratio of 3 and silica sol was added to get a SiO2/A2O3 ratio of 37.2 in the hydrothermal gel.
Reanvattana, (2005) synthesised high purity zeolite ZSM-5 from a mixture of fused fly ash filtrate and added silica extracted from rice husk ash.
Chareonpanich et al., (2004) synthesised zeolite ZSM-5 from Thailand fly ash with SiO2/Al2O3 ratio of 1.6. SiO2 extracted from rice husk ash was added to adjust the Si:Al molar ratio to 20, 40, 60, 80, 100 or 200. The addition of an important amount of silica source in the hydrothermal gel and the presence of unreacted fly ash in the final product after synthesis did not promote the synthesis of high silica content zeolites as a way to valorise fly ash, and has to date limited the commercialisation and applications of fly ash-based zeolite ZSM-5.
Therefore, there remains a need to develop a commercially attractive method for the synthesis of aluminosilicate zeolite materials, in particular high silica content zeolites such as ZSM-5, from fly ash as an alternative to the use of relatively high cost pure starting materials.